c# print barcode zebra printer Lesson 1: Exploring Specialized Web Server Controls in .NET framework

Generating Quick Response Code in .NET framework Lesson 1: Exploring Specialized Web Server Controls

Lesson 1: Exploring Specialized Web Server Controls
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Figure 3-12 The rendered Calendar control displaying special days and having a selected date.
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The FileUpload Control
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The FileUpload control is used to display a TextBox and Browse button that allows a user to either type a file name and path, or click Browse and select a file and path. This control generates an <input type="file"> element when rendering to HTML. The FileUpload control inherits directly from the WebControl class, as shown in Figure 3-13. The FileUpload control is represented as the <asp:FileUpload> element in source view and has no content, so you can write this element as a singleton element. The FileUpload control does not cause a PostBack to the Web server. After the user selects a file, the user needs to cause a PostBack via a different control, such as a Button. The PostBack causes the file to be uploaded to the server as posted data. At the server, the page code does not run until the file is uploaded to server memory.
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Exploring Specialized Server Controls
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WebControl Class Control
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FileUpload Class WebControl Properties FileBytes FileContent FileName HasFile PostedFile Methods AddAttributesToRender FileUpload OnPreRender Render SaveAs
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Figure 3-13 The FileUpload control hierarchy.
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The following properties give you flexible ways to access the uploaded file:
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FileBytes
The file is exposed as a byte array. The file is exposed as a stream.
FileContent PostedFile
The file is exposed as an object of type HttpPostedFile. This object has properties, such as ContentType and ContentLength properties.
You need to examine any file that is uploaded to determine if it should be saved; you can examine characteristics such as the file name, size, and MIME type. MIME stands for Multipurpose Internet Mail Extensions and specifies the type of file that is being uploaded. When you re ready to save the file, you can use the SaveAs method on the FileUpload control or the HttpPostedFile object. You can save the file in any location for which you have permissions to create files. By default, the requireRootedSaveAsPath attribute of the httpRuntime configuration ele ment in the Web.config file is set to true, which means that you need to provide an absolute path to save the file. You can get an absolute path by using the MapPath method of the HttpServerUtility class and passing to the method the tilde (~) operator, which represents the application root folder.
Lesson 1: Exploring Specialized Web Server Controls
The maximum size file that can be uploaded depends on the value of the MaxRequestLength attribute of the httpRuntime configuration element in the Web.config file. If users attempt to upload a file that is larger than the MaxRequestLength, the upload fails.
Security Alert
The FileUpload control allows users to upload files but makes no attempt to vali date the safety of the uploaded files. The FileUpload control doesn t provide a means to filter the file types that can be uploaded by a user, but you can examine the file characteristics, such as the file name and extension, as well as the ContentType, after the file has been uploaded. Although you can provide client-side script to examine the file that is being submitted, remember that client-side validation is a convenience for the honest user. A hacker can easily strip the Web page of client-side code to bypass this validation.
In this example, a Web page was created and a FileUpload control was added to the page. In addition, a Button was added to the Web page that is used to submit the file to the Web server via PostBack. A folder was added to the Web site called Uploads. The following code was added to the code-behind page to show how the FileUpload con trol s properties can be set programmatically, and a file can be uploaded and saved.
'VB Partial Class FileUpload_Control Inherits System.Web.UI.Page Protected Sub Button1_Click(ByVal sender As Object, _
ByVal e As System.EventArgs) Handles Button1.Click
If (FileUpload1.HasFile) Then
Label1.Text = "File Length: " _
+ FileUpload1.FileBytes.Length.ToString() _ + "<br />" _ + "File Name: " _ + FileUpload1.FileName _ + "<br />" _ + "MIME Type: " _ + FileUpload1.PostedFile.ContentType
FileUpload1.SaveAs( _
MapPath("~/Uploads/" + FileUpload1.FileName))
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